(CWG)

* Winter storm warning for western Loudoun, northern Montgomery, western Howard and Frederick counties, and to the north and west noon Sunday to 4 a.m. Monday | Winter weather advisory for northern Fauquier, eastern Loudoun, southern Montgomery and eastern Howard counties from noon Sunday to 4 a.m. Monday *

9:45 p.m. — First weather models of the evening shift rain/snow line somewhat north

There wasn’t much change in the first set of model runs tonight. That said, the gradient — from several inches of accumulating snow to little or none — shifted somewhat north of where it was earlier.

This shift in the gradient was accompanied by temperatures about a degree warmer at the ground on average. When we’re talking readings near or just above freezing, these small changes can matter a good deal. Living on the edge is tough.

We are not going to change our forecast map at this point, but we may do so before the event begins. If anything, this newer data suggests totals on the lower end of the first call, although that is less certain the farther north you go into colder temperatures and higher elevation. These small bumps have also gone south at times, so we can’t totally rule another correction that way either. Certainly a fluid situation, at least in the immediate area.

Original article....

A storm system sweeping northeast along an Arctic front promises to bring a messy mix of winter weather to the region on Sunday. This is an event in which conditions will steadily deteriorate as you head north and northwest of the District, and conditions may vary significantly over rather small distances.

Precipitation is expected to begin between midmorning and midday Sunday and end a little after midnight.

During the peak of this storm, from late Sunday afternoon and evening, the rain-snow line will straddle the Washington region. Areas to the north of it may see disruptive amounts of snow, while to the south it should mostly be just a cold rain. In the middle, it should be a mix and, while some slushy accumulation is possible on grassy areas, paved surfaces should stay mostly clear.

The heaviest accumulations of three to six inches are expected from central Loudoun County through northern Howard and Montgomery counties and to the north and west. Heavier amounts are possible in northern Maryland near the Pennsylvania border. Untreated roads could turn hazardous in this area, especially by late afternoon and early evening Sunday. Mostly snow should fall in this zone, although we can’t rule out a little sleet Sunday evening, especially in the southern portion.

In Washington’s close-in northern and western suburbs, from central Fairfax County through southern Montgomery and northern Anne Arundel counties, one to three inches are most likely, mainly on grassy areas. Here, the snow could mix with and change to sleet and/or rain for a time Sunday evening.

From Prince William County through southern Fairfax County, the District, central Prince George’s County to around Annapolis, a coating of up to two inches of snow is forecast, mainly on grassy areas. In this region, the snow has a good chance to mix with or change to sleet and rain at times, especially late Sunday afternoon into the evening.

Note that small changes in the track of the storm could shift the rain-snow line south or north, which would increase or decrease snow amounts in the immediate metropolitan region.

Throughout the region, even where we expect more than a couple inches of snowfall, temperatures are predicted to be only marginally cold enough for accumulation. “Surface temperatures for much of the storm are forecast to be a degree or two above freezing,” said Wes Junker, Capital Weather Gang’s winter weather expert. “These temperatures, along with the strength of the March sun Sunday afternoon, are likely to limit snowfall accumulations.”

When the snow is not falling steadily, it will have a hard time accumulating except on grassy areas, which will reduce the impact some. But during heavier bursts, visibility will drop and roads could become slushy even for brief times in the immediate area.

Elevation will play a big role in snow amounts, which is typical for March storms, noted Junker. Locations with at least 500 or 750 feet of elevation in our west and northwest areas will be colder and will see more snow accumulate than low-lying areas.

Storm timeline


American model forecast for evolution of precipitation Sunday and Sunday night. It is among the snowier models for Washington. (PivotalWeather.com/PivotalWeather.com)

9 a.m. to noon Sunday: Areas of light snow (north) and mixed precipitation (south) develop from west to east. No accumulation. Temperatures 35 to 39 degrees (northwest to southeast)

Noon to 4 p.m. Sunday: Precipitation increases in coverage and intensity. More snow north and west of the Beltway and a mix of snow, sleet and rain elsewhere, with mostly rain in southern areas. Some accumulation, mainly on grassy areas, should begin where it’s snowing. Temperature falling to 32 to 37 degrees (northwest to southeast)

4 p.m. to 8 p.m. Sunday: Snow north and west of the Capital Beltway, possibly mixing with sleet. Wintry mix or rain from the Beltway south (more rain likely south of Fairfax County). Snow accumulation likely in colder areas north and west of the Beltway, some slick spots possible. Elsewhere, where there’s snow and sleet, most accumulation is on grassy areas except during heavier bursts. Temperatures 31 to 36 degrees (northwest to southeast).

8 p.m. Sunday to 12 a.m. Monday: Snow north and west of the Beltway, possibly mixing with sleet or even rain at lower elevations. Wintry mix around the Beltway and south may change to plain rain. Additional snow accumulation and slick spots in colder areas north and west of the Beltway. Elsewhere, where there’s snow and sleet, most accumulation is on grassy areas except during heavier bursts. Temperatures 31 to 36 degrees (northwest to southeast).

12 a.m. to 3 a.m. Monday: Precipitation rapidly tapers off from southwest to northeast. Some areas with rain and wintry mix may change briefly back to snow. Little additional or new accumulation. Temperatures 30 to 35 degrees (northwest to southeast).


NAM model forecast of precipitation Sunday and Sunday night. It is among the rainier models for Washington. (PivotalWeather.com/PivotalWeather.com)

Impact rating

On our winter storm impact scale, this storm rates as a Category 1 “nuisance” storm for the immediate area — within a one-county radius of Washington, except for northern Montgomery County, where it rates as a Category 2 “disruptive” storm. It also rates a Category 2 in Loudoun, Frederick and western Howard counties and to the north and west of those areas.


(CWG)

Category 2 storms cause slick roads and, in this instance, may prompt school delays and closings on Monday in affected areas.

Category 1 storms may cause pockets of slick travel during heavy bursts of frozen precipitation, but paved surfaces mostly remain wet with accumulation mostly on grassy areas.

South of Fairfax County, the storm does not earn a rating.

The impact of this storm will be limited throughout the region by temperatures that are just marginally cold enough for snow and by its timing over the weekend rather than during a commuting period.

Model forecasts

As you might imagine with the rain-snow line straddling the region, model forecasts for Washington, right along that line, vary widely. Here’s what they project for the District:

  • High-resolution NAM: 0.2 inches
  • NAM: 0.8 inches
  • European: 2 inches
  • GFS: 2 inches
  • HRRR: 4.1 inches
  • High-resolution Canadian: 4.1 inches
  • Canadian: 5.4 inches

We lean toward the lower end of these projections in the immediate area because temperatures are forecast to be a little above freezing, which means some of the snow will melt rather than accumulate. Plus, any sleet and rain mixing with the snow will tend to hold back accumulation.